1. Field of the Invention
The present invention relates in general to an apparatus for controlling a drive system for an automotive vehicle, which drive system includes a drive power source and an automatic transmission. More particularly, the invention relates to improvements in techniques for increasing an output of the drive power source so as to prevent a shifting shock of the automatic transmission, upon a shift-down action of the automatic transmission while the vehicle is in a coasting run without a drive force being transmitted from the drive power source to vehicle drive wheels.
2. Discussion of Related Art
There is widely known an automotive vehicle provided with a drive system including a drive power source such as an internal combustion engine, a fluid-operated power transmitting device connected to the drive power source, and an automatic transmission arranged to transmit a rotary motion of the fluid-operated power transmitting device to drive wheels such that a ratio of the input speed of the automatic transmission to its output speed is variable. An example of such a vehicle is disclosed in JP-A-7-139382 wherein a torque converter provided as the fluid-operated power transmitting device is connected to an automatic transmission such as a planetary gear type transmission. The automatic transmission has a plurality of operating positions which have respective different speed ratios and which are selectively established by respective combinations of engaging and releasing actions of a plurality of frictional coupling devices. In such a vehicle, the automatic transmission is shifted down to increase its speed ratio during a coasting run of the vehicle without a drive force being transmitted from the drive power source to the vehicle drive wheels, with an accelerator pedal being held in its non-operated position, for example. An increase in the speed ratio causes an increase in the operating speeds of the fluid-operated power transmitting device and the drive power source. If this increase in the operating speeds takes place in a so-called xe2x80x9creverse power transfer statexe2x80x9d while the speed of the drive power source is lower than the output speed of the fluid-operated power transmitting device (which is the input speed of the automatic transmission), there arise a relatively large amount of variation in the drive force due to an inertia of the drive power source, and application of a relatively large load to the frictional coupling devices and other elements of the automatic transmission. In view of these drawbacks, it is considered to positively increase the output and operating speed of the drive power source upon a shift-down action of the automatic transmission. It is also considered to hold the operating speed of the drive power source during the coasting run of the vehicle higher than the output speed of the fluid-operated power transmitting device (the input speed of the automatic transmission), so that the vehicle is driven with a high response to a subsequent operation of the accelerator pedal which takes place at the end of the coasting run. If the speed of the drive power source is increased in the reverse power transfer state while the speed of the drive power source is lower than the output speed of the by fluid-operated power transmitting device, the direction in which a torque is transmitted through the fluid-operated power transmitting device is reversed, so that the fluid-operated power transmitting device is likely to suffer from a shock. To avoid this drawback, it is desirable to positively increase the operating speed of the drive power source upon a shift-down action of the automatic transmission during the coasting run of the vehicle.
If the output of the drive power source is increased to positively increase its operating speed in the reverse power transfer state while the speed of the drive power source is lower than the output speed of the fluid-operated power transmitting device, the speed of the drive power source is abruptly raised due to a comparatively small load acting on the drive power source, so that the direction in which the torque is transmitted through the fluid-operated power transmitting device is suddenly reversed, giving rise to a risk of generation of noise and shock. The vehicle drive system may be placed in the reverse power transfer state, when a fuel-cut control to cut a fuel supply to the drive power source is effected under a predetermined condition, to improve the fuel economy of the vehicle or to apply a drive-power-source brake such as an engine brake to the vehicle, and when any device (e.g., an air conditioning system) operable by the drive power source is activated.
The present invention was made in the light of the background art discussed above. It is therefore an object of the present invention to provide an apparatus for controlling a vehicle drive system including a drive power source, a fluid-operated power transmitting device and an automatic transmission, which apparatus is capable of reducing operating noise and shock that may be generated due to a change in the direction of transmission of a torque through the fluid-operated power transmitting device, when the output of the drive power source is increased to increase its operating speed upon a shift-down action of the automatic transmission during a coasting run of the vehicle in which the speed of the drive power source is lower than the output speed of the fluid-operated power transmitting device,
The object indicated above may be achieved according to the principle of the present invention, which provides an apparatus for controlling a drive system of an automotive vehicle including a drive power source, a fluid-operated power transmitting device connected to the drive power source, and an automatic transmission arranged to transmit a rotary motion of the fluid-operated power transmitting device to drive wheels of the vehicle such that a speed ratio which is a ratio of an input speed of the automatic transmission to an output speed thereof is variable, the apparatus being arranged to increase an output of the drive power source for increasing an operating speed of the drive power source upon a shift-down action of the automatic transmission so as to increase the speed ratio during a coasting run of the vehicle in which the output of the drive power source is not transmitted to the drive wheels, the apparatus comprising: an output controlling element operable to control the output of the drive power source; and gradually output increasing means operable upon the shift-down action of the automatic transmission during the coasting run of the vehicle, for controlling the output controlling element so as to gradually increase the operating speed of the drive power source at a predetermined rate.
In this vehicle drive system control apparatus, the output controlling element is controlled by the gradually output increasing means, to gradually increase the operating speed of the drive power source at the predetermined rate, so that a shock to be generated upon the shift-down action of the automatic transmission can be effectively reduced. Where the gradually output increasing means is operated in a reverse power transfer state of the vehicle drive system in which the speed of the drive power source is lower than the output speed of the fluid-operated power transmitting device, the gradual increase of the speed of the drive power source permits a gradual or slow reversal of the direction in which a torque is transmitted through the fluid-operated power transmitting device, so that the amounts of noise and shock due to the reversal of the torque transmitting direction are effectively reduced.
As the drive power source, an internal combustion engine such as a gasoline engine and a diesel engine may be suitably used. However, a drive power source of any other type may be used. As the fluid-operated power transmitting device, a torque converter and a fluid coupling may be suitably used. The automatic transmission may be preferably constituted by a planetary gear type transmission including a plurality of frictional coupling devices such as clutches and brakes, which are selectively engaged and released to selectively establish a plurality of operating positions having respective different speed ratios. However, the automatic transmission may be other types of multiple-step transmission, such as a two-axes synchronous coupling transmission which is shifted by hydraulic and other actuators, or a continuously variable transmission of belt-and-pulley type whose speed ratio is continuously variable.
According to a first preferred form of the apparatus of this invention, the apparatus further comprises reverse power transfer determining means operable upon the shift-down action of the automatic transmission during the coasting run of the vehicle, for determining whether the drive system is placed in a reverse power transfer state in which the operating speed of the drive power source is lower than an output speed of the fluid-operated power transmitting device, and the gradually output increasing means is operated to control the output controlling element so as to gradually increase the operating speed of the drive power source, when the reverse power transfer determining means has determined that the drive system is placed in the reverse power transfer state.
In the apparatus according to the first preferred form of the present invention described above, the gradually output increasing means is operated when the reverse power transfer determining means has determined that the drive system is placed in the reverse power transfer state. In the reverse power transfer state, therefore, the output controlling element is controlled to gradually increase the speed of the drive power source at the predetermined rate, so that the direction of the torque transmission through the fluid-operated power transmitting device is gradually or slowly reversed, whereby the amounts of noise and shock due to the reversal of the torque transmitting direction are accordingly reduced.
During the coasting run of the vehicle, the output of the drive power source is not transmitted to the vehicle drive wheels to drive the vehicle, usually with the accelerator pedal placed in its non-operated position. During this coasting run, the vehicle drive system is generally placed in the reverse power transfer state in which the operating speed of the drive power source is lower than the output speed of the fluid-operated power transmitting device (which is equal to the speed of the input shaft of the automatic transmission). The vehicle control apparatus of the present invention is effective and advantageous when the gradually output increasing means is operated when the reverse power transfer state of the vehicle drive system is detected by the reverse transfer determining means, as in the first preferred form of this invention described above. In particular, the reverse power transfer state is established when a fuel cut control is effected to improve the fuel economy of the vehicle or to apply a drive-power-source brake to the vehicle in the coasting run of the vehicle. However, the operation of the gradually output increasing means to gradually increase the speed of the drive power source by controlling the output controlling element is effective to reduce the shifting shock, also when the operating speed of the drive power source is held higher than the output speed of the fluid-operated power transmitting device, for enabling the vehicle to be driven with a high response to a subsequent operation of the accelerator pedal which takes place at the end of the coasting run of the vehicle. However, the speed of the drive power source may be rapidly or instantaneously increased to a predetermined target value, when the shift-down action of the automatic transmission is detected or expected to be effected while the speed of the drive power source is higher than the output speed of the fluid-operated power transmitting device. In this case, the gradually output increasing means is not operated.
According to a second preferred form of the present invention, the gradually output increasing means is operated irrespective of whether the drive system is placed in a reverse power transfer state in which the operating speed of the drive power source is lower than an output speed of the fluid-operated power transmitting device. The vehicle drive system may arranged to be placed in the reverse power transfer state when the automatic transmission is shifted down during the coasting run of the vehicle. In this case, the gradually output increasing means is necessarily operated when the shift-down action of the automatic transmission is detected during the coasting run. The vehicle drive system may be arranged to avoid the reverse power transfer state upon a shift-down action of the automatic transmission during a coasting run of the vehicle. In this case, too, the gradually output increasing means is necessarily operated when the automatic transmission is shifted down during the coasting run.
According to a third preferred form of the invention, the output controlling element is operable to change the output of the drive power source, irrespective of an operation of a vehicle accelerating member such as an accelerator pedal by an operator of the vehicle. For example, the output controlling element is an idling speed control valve operable to change an idling speed of the drive power source. Alternatively, the output controlling element is an electronic throttle valve which is controlled by a throttle actuator controllable by the present apparatus. Where the vehicle accelerating member such as an accelerator pedal is mechanically connected to a mechanical throttle valve, the output of the drive power source can be changed by controlling the idling speed control valve, irrespective of an operation of the vehicle accelerating member.
The gradually output increasing means may be adapted to control the output controlling element such that the operating speed of the drive power source is increased at a predetermined constant rate. The gradually output increasing means may be adapted to control the output controlling element such that the operating speed of the drive power source is increased to a predetermined constant value. Alternatively, the gradually output increasing means controls the output controlling element such that the operating speed of the drive power source is increased at a rate which is determined by a kind of the shift-down action of the automatic transmission. Similarly, the gradually output increasing means may be adapted to control the output controlling element such that the operating speed of the drive power source is increased to a target value which is determined by the kind of the shift-down action of the automatic transmission. Further, the gradually output increasing means may be arranged to control the output controlling element such that the operating speed of the drive power source is increased at a rate which is determined by a difference between the operating speed of the drive power source and an output speed of the fluid-operated power transmitting device. Similarly, the gradually output increasing means may be arranged to control the output controlling element such that the operating speed of the drive power source is increased to a target value which is determined by the difference between the operating speed of the drive power source and an output speed of the fluid-operated power transmitting device. The operating speed of the drive power source need not be linearly increased at a constant rate. Namely, the gradually output increasing means may be adapted to control the output controlling element such that a rate of increase of the operating speed of the drive power source continuously varies. Alternatively, the gradually output increasing means may be adapted to control the output controlling element such that the operating speed of the drive power source is initially increased by an initial increase amount which is determined by at least one of the kind of the shift-down action of the automatic transmission and the difference between the operating speed of the drive power source and the output speed of the fluid-operated power transmitting device. In this case, the initial increase amount may be determined on the basis of at least one of the kind of the shift-down action and the speed difference and according to a stored data map or a predetermined equation representative of a predetermined relationship between the initial increase amount and the above-indicated at least one of the kind of the shift-down action and the speed difference. Alternatively, the gradually output increasing means may be adapted to control the output controlling element such that the operating speed of the drive power source is increased at a rate which is determined by the above-indicated at least one of the kind of the shift-down action and the speed difference. In this case, the rate of increase of the operating speed of the drive power source may be determined on the basis of the at least one of the kind of the shift-down action and the speed difference and according to a stored data map or a predetermined equation representative of a predetermined relationship between the rate of increase and the at least one of the kind of the shift-down action and the speed difference.